The present invention relates to a partial discharge measuring a device for measuring partial discharge pulse produced in a gas insulated electric apparatus such as gas insulated, sealed circuit breaker, gas insulated cable or gas insulated transformer or a high voltage apparatus such as high voltage rotary electric machine of solid insulative device of such as resin molded transformer, separately from electric noise and, particularly, to such measuring device having a noise remover circuit for reducing or removing single shot noises or periodic noise included in the partial discharge pulse and having a rising and/or falling time longer than a duration of the pulse.
When a characteristics of a sample apparatus is to be evaluated by measuring or observing a waveform, amplitude and/or frequency of occurence of the partial discharge pulse signal and if a level of noise such as periodic noise due to broadcasting wave introduced, through the apparatus acting as an antenna into a measuring circuit, on-off noise introduced through power transmission system into the measuring circuit or single shot noise such as electric discharge noise is high, an S/N ratio of the measuring circuit etc. is lowered and hence the evaluation accuracy is degraded.
As a noise removing circuit, it has been usual to use a filter having a passband in which main frequeency components of the pulse signal is included. When the passband is made narrower to improve the S/N ratio, the pulse signal waveform may be distorted or vibrate and when it is made wider, the S/N ratio for noise having a waveform which is closer to that of the main frequency component of the pulse signal is lowered. That is, it is practically impossible to obtain a noise removing circuit having a characteristic which satisfies the noise removal and where the original waveform maintains its performance.
FIG. 1 is a circuit construction of a conventional noise removing circuit which is applied to a partial discharge testing circuit for a high voltage electric device such as power cable. In FIG. 1, a reference numeral 1 depicts a power cable which is to be tested, 2 a terminal bushing for applying a test voltage to the power cable 1, 3 a coupling capacitor for partial discharge pulse detection, 4 a detection impedance, 5 a noise removing circuit composed of a delay line 6 and an adder circuit 7 and 8 an amplifier for amplifying the partial discharge pulse.
FIG. 2 shows signal waveforms for explaining an operation of the noise removing circuit. In FIG. 2, references W4, W6 and W7 depict an output signal waveform (terminal voltage at the detection impedance 4) of an input circuit 10, an output signal waveform of the delay line 6 and an output signal waveform of the adder 7, respectively. Also in FIG. 2, the signal W4 includes a partial discharge pulse P1 which is to be detected and a periodic noise N1 having a period .tau.. The delay line 6 is designed to have a delay time corresponding to a half (.tau./2) of the period of the periodic noise N1 so that the output waveform W6 includes a noise N and discharge pulse P2 which are delayed from the input signals N1 and P1 by .tau./2, respectively. As a result, the phase of the noise components N1 and N2 of the input signals W4 and W6 to the adder 7 are opposite to each other and cancelled out by the adder 7 and only the discharge pulses P1 and P2 are supplied to the amplifier 8 provided at a output terminal 9 after amplified, as shown by the waveform W7.
In this circuit construction, since the periodic noise is delayed by a half period, it is necessary to regulate the delay time of the delay line 6 such that it coincides with the period of the periodic noise. In addition, when a plurality of periodic noises exist, a corresponding number of noise removing circuits are required.